The present invention relates to an optical fiber connector system. More particularly, the present invention relates to a connector assembly for optically coupling a circuit card to a backplane.
The use of optical fibers for high-volume high-speed communication is well established. As the volume of transmitted information grows, the use of optical fiber cables including multiple optical fibers, and of systems using multiple optical fiber cables, has increased.
It has long been desirable to increase the number of fibers that can be removably connected within a given space. Until recently fiber optic interconnects were limited to single or duplex formats utilizing industry standard connectors, such as the SC, ST, LC, and the like. These solutions are analogous to single end electrical cable terminations prevalent prior to the invention of electrical ribbon cable and mass-terminable IDC connectors.
Fiber optic terminations currently are evolving from single terminations to mass terminations. Within the past few years, ribbonized multi fiber cables have been developed. In conjunction with these cable development efforts, multi-fiber mounting ferrules also have been developed.
The design of traditional electronic cabinets is now being utilized to accommodate optical and opto-electronic devices. In traditional cabinet designs, the cabinet comprises a box having a plurality of internal slots or racks, generally parallel to each other.
Components are mounted on planar substrates, called as circuit boards or daughter cards, which are designed to slide into the slots or racks within the cabinet.
As with electrical cables, the need exists to provide a means to allow the fiber signals to be passed through the backplane of electronic cabinets. A backplane derives its name from the back (distal) plane in a parallelepipedal cabinet and generally is orthogonal to the board cards. The term backplane in the present invention refers to an interconnection plane where a multiplicity of interconnections may be made, such as with a common bus or other external devices. For explanation purposes, a backplane is described as having a front or interior face and a back or exterior face.
An example of a backplane connectivity application is the interconnection of telephone switching equipment. In this application, cards having optical and electronic telecommunication components are slid into cabinets. The need exists to have a removable fiber termination from both the front side and the back-side of the backplane.
Furthermore, as a function of inserting and removing an optical driver card from a rack coupled to the backplane, coupling and uncoupling of the optical connections in the card is to be completed in a blind mating manner.
In order to maintain appropriate transmission of light signals, optical fiber ends are to be carefully aligned along all three movement (x, y, and z) axes, as well as angularly. Alignment challenges increase and tolerances decrease geometrically as the number of optical fibers to be aligned increases. Blind mating of a card-mounted component to a backplane connector has been found to create special challenges with regards to alignment and mating force issues along the axis of interconnection.
For the purposes of the present description, the axis of interconnection is called the longitudinal or x-axis and is defined by the longitudinal alignment of the optical fibers at the point of connection. Generally, in backplane applications, the longitudinal axis is collinear with the axis of movement of the cards and the axis of connection of the optical fibers in and out of the cabinets. The lateral or y-axis is defined by the perpendicular to the x-axis and the planar surface of the card. Finally, the transverse or z-axis is defined by the orthogonal to the x-axis and the backplane surface. The angular alignment is defined as the angular orientation of the card with respect to the x-axis.
In preferred embodiments, the motion of sliding the card into a receiving slot simultaneously achieves optical interconnection. The “optical gap” distance along the longitudinal axis between the optical fiber ends and interconnected optical components is an important consideration. A large gap will prevent effective connection, thereby causing the loss of the optical signals. On the other hand, excessive pressure on the mating faces, such as that caused by “jamming in” a card, may result in damage to the fragile optical fiber ends and mating components. Traditional optical gap tolerances are in the order of less than one micron.
Current connector assemblies include forward biased spring mounted ferrules. The purpose of the said bias springs is twofold, one, to absorb a limited amount of over travel of the ferrules during mating and two, to provide a predetermined spring biasing force thus urging the ferrules intimately together when the ferrules are in their mated position.
An additional subject of concern is card gap, especially when dealing with backplane connector systems. Card gap is defined as the space remaining between the rear edge of a circuit card and the interior or front face of the backplane. In general, designers and users of backplane connection systems find it exceedingly difficult to control the position of a circuit card to a backplane within the precision range required for optical interconnects. Card gap, otherwise defined as card insertion distance, is subject to a multiplicity of variables. Among these variables are card length, component position on the surface of the card, card latch tolerances, and component position on the backplane.
Over insertion of a circuit card relative to the interior surface of a backplane presents a separate set of conditions wherein the backplane connector's components are subjected to excessive compressive stress when fixed in a mated condition. In certain instances the said compressive stress may be sufficient to cause physical damage to the connector's components and the optical fibers contained therein.
The need remains for a connector system that prevents component damage due to excessive operator force, compensates for longitudinal card misalignment, yet provides accurate control of optical gap distance and mating force.
Another consideration is radial misalignment of the card. When an operator inserts a card on a slot, it is often difficult to maintain the card edge perfectly aligned in parallel with the lateral axis of the backplane. FIG. 1 illustrates an angularity misaligned card 10 having a connector 12 mating to a backplane connector 14. The card is otherwise correctly aligned along the y and z-axes. At the point of contact between connectors 12 and 14, the angular misalignment prevents correct gap spacing between optical fibers 16 and causes undue pressure on one end of the connector and the respective optical fiber end faces.
Other considerations exist in backplane interconnection systems other than correct alignment. With the advent of laser optical signals and other high-intensity light sources, eye safety is a major concern associated with backplane connector users today. The safety issues are further escalated by the fact that ribbonized fiber arrays present a greater danger than the single fiber predecessors because the amount of light is multiplied by the number of fibers.
Previous systems, such as that discussed in U.S. Pat. No. 5,080,461, discuss the use of complex door systems mounted on terminating fiber connectors, but mainly for the purpose of preventing damage or contamination of fiber ends. As the lighttransmitting core of a single mode fiber measures only ˜8 microns in diameter, even a minute accumulation of dust particles may render the fiber inoperable. However, prior systems require complex terminations at each fiber end and only may be mated to another corresponding male-female connector pair, not to standard connectors, making their use cumbersome.
EMI (electromagnetic interference) control also has arisen as an issue in backplane connector design. As connection of optoelectronic devices through a backplane often necessitates forming of a physical opening through the backplane of an electronic cabinet, the potential exists for EMI leakage through the said backplane. Electrical interconnection has attempted to address this problem through the use of several elaborate EMI shielding techniques. However, current optical fiber connectors have failed to satisfy this concern.
Finally, another concern regarding backplane optical connector applications is bend radius control. Horizontal cabinets connections are often subject to bend stresses due to gravity, operator misuse, or physical constraints, such as when a cabinet is pressed against a wall. Optical fibers are made of glass and rely on total internal reflection to transmit light signals. When an optical fiber is bent beyond a certain critical angle, fractures may appear in the glass, causing the fiber to break or become damaged. Also, at certain bend angles, even if the glass fiber does not break, the optical signal may be lost or may deteriorate, as the complete light signal is no longer contained inside the fiber.
Several methods and apparatus for controlling the bend radius of an optical cable have been attempted. Among those are preformed boots that are slid over the cable, external devices such as clips or clamps, and elaborate injection molded components that are shaped such that when attached to a cable, the cable assumes the shape of the molded structure.
Since backplane connection frequently involves connecting an increasing number of optical fibers in a small space, the need exists for an apparatus for controlling the bend radius of the optical fibers.